12
The input stage of the typical application circuit in Figure
19 can be simpli ed as the diagram shown in Figure 20.
R2 and R
IN
, input resistance of the ACPL-C87X, create a
current divider that results in an additional measurement
error component that will add on to the tot on top of the
device gain error. With the assumption that R1 and R
IN
have a much higher value than R2, the resulting error can
be estimated to be R2/R
IN
.
With R
IN
of 1 G for the ACPL-C87X, this additional mea-
surement error is negligible with R2 up to 1 M, where the
error is approximately 0.1%. Though small, it can be further
reduced by reducing the R2 to 100 k (error of 0.01%
approximately), or 10 k (error of 0.001% approximately).
However with lower R2, a drawback of higher power dis-
sipation in the resistive divider string needs to be consid-
ered, especially in higher voltage sensing applications. For
example, with 600 V DC across L1 and L2 and R2 of 100 k
for 0.01% measurement error, the resistive divider string
Figure 20. Simpli ed Input Stage.
Figure 21. Thermistor sensing in IGBT Module
consumes about 12 mW, assuming V
IN
is set at 2 V. If the R2
is reduced to 10 k to reduce error to 0.001%, the power
consumption will increase to about 120 mW. In energy
e ciency critical applications such as PV inverters and
battery-powered applications, this trade-o between
measurement accuracy and power dissipation in the
resistive string provides exibility in design priority.
Isolated Temperature Sensing using Thermistor
IGBTs are an integral part of a motor or servo drive system
and because of the high power that they usually handle,
it is essential that they have proper thermal management
and are su ciently cooled. Long term overload conditions
could raise the IGBT module temperature permanently or
failure of the thermal management system could subject
the module to package overstress and lead to catastrophic
failures. One common way to monitor the temperature
of the module is through using a NTC type thermistor
mounted onto the IGBT module. Some IGBT module man-
ufacturers also have IGBTs that comes with the thermistor
integrated inside the module. In some cases, it is necessary
to isolate this thermistor to provide added isolation and
insulation due to the high power nature of the IGBTs. The
ACPL-C87x voltage sensor can be used to easily meet
such a requirement, while providing good accuracy and
non-linearity. Figure. 21 shows an example of such an
implementation. The ACPL-C87x is used to isolate the
thermistor voltage which is later fed by the post amp
stage to an ADC onboard the microcontroller (MCU) to
determine the module temperature. The thermistor needs
to be biased in way that its voltage output will optimize
the 2 V input range of the ACPL-C87x across the intended
temperature measurement range.
Measurement Accuracy and Power Dissipation of the Resistive Divider
+ –
+
GND
ACPL-C87x
R1
R2
R
IN
HV+
HV-
IGBT Module
NTC Thermistor
U
V
W
+
–
+
GND
ACPL-C87x
Vdd
MCU
ADC
Post
Amp
